Drilling a wellbore is often performed using drilling fluid that is maintained at a density so as to create a hydrostatic pressure that exceeds the pore-pressure of connate fluids in any formation through which the wellbore penetrates. This drilling condition is known as “over-balanced” and can cause “leak-off” or “lost circulation” of the drilling fluid into the formation. Lost circulation can be a significant issue while drilling. Drilling muds typically contain particles such as calcium carbonate to restrict leak-off. The particles typically form a “filtercake” or “mudcake” on the wellbore wall that presents a low-permeability barrier to any additional losses in a given section of the wellbore. This filtercake is typically deliberately designed to be weak or low-strength so that when the well is allowed to flow back with hydrocarbons, the mudcake cleans up easily and does not restrict the flow of hydrocarbons into the wellbore. Any restriction would be considered part of the near-wellbore formation damage or reservoir “skin” and would result in a lower productivity index, which is highly undesirable for an operator. However, the fragile nature of the filtercake typically means that it is constantly reformed as it is scraped off by the passing drillstring, drill-stabilizers, or directional drilling pads.
In non-reservoir, (typically) upper regions of the wellbore where there are no recoverable hydrocarbons, it is generally desirable to seal this region permanently. This is typically achieved at several intervals by placing and cementing casing. Casing resolves the issue of leak-off and accommodates changes in mud-weight (i.e., density) for pore-pressure control. However, cementing on top of filtercake is a difficult process, and improper filtercake cleanup results in a poor quality cement bond between the formation and the casing. Where a poor bond exists, well-control issues become a risk since the poor bond can present an uncontrolled channel to the surface.
Drilling in regions with high tectonic stresses may require significant over-balance to maintain wellbore stability, while still encountering regions of significant porosity and regions of low pore-pressure such as when an area has been producing for some time or in the presence of heterogeneity. In these formations the need to counter a high level of leak-off, together with the mechanical damage to and subsequent rebuilding of the filtercake, can result in a significant volume of solids being deposited on the wellbore wall, thereby producing a thick filtercake. This can lead to “differential sticking”, whereby the drillstring embeds in the filtercake material and is held in place by the differential pressure between the over-balance fluid pressure and the significantly lower formation pore-pressure. Such differential sticking causes very significant non-productive time.